Conventional ink jet printing systems use various different methods to produce ink droplets directed toward a recording medium. Well known devices for ink jet printing include thermal, piezoelectric, and acoustic ink jet print heads. All of these technologies produce roughly spherical ink droplets having a 15-100 xcexcm diameter directed toward a recording medium at approximately 4 m/sec. The ejecting transducers or actuators in the print heads, which produce the ink droplets, are controlled by a printer microcomputer or controller. The printer controller activates the transducers or actuators in conjunction with movement of the recording medium relative to the print head. By controlling the activation of the transducers or actuators and the recording medium movement, the printer controller directs the ink droplets to impact the recording medium in a specific pattern, thus forming an image on the recording medium.
In devices of the type described above, there is need for a mechanism to hold and to advance the print medium during the course of creating images. This requirement is necessary to control media motion and hence image quality. The conventional means resort to vacuum hold-down whereby suction is created between the print medium and the print support by drawing air through small orifices on the support plate. This technique suffers from several disadvantages: the system is noisy with the use of a compressor; power consumption is high; and most critical of all, the airflow creates a disturbance to the drop trajectories leading to errors in drop placement that adversely affect print quality.
Electrostatic methods offer an improved tacking mechanism. The conventional approach is to use corona devices to spray charge onto dielectric surfaces to form the holding force. Two major disadvantages are: the residual charge needs to be neutralized to prevent static shock from contact with the transport surfaces, and the use of corona devices lead to ozone production which requires venting of the surrounding environment. A more viable alternative proposed in this invention is the use of fringe fields, which do not involve static charge and therefore charging devices. These fields are easily turned on and off and are sustained by application of low voltage to electrodes, which are embedded beneath the print medium. Therefore static shock is no longer a problem. A further advantage is that this method allows distributed tacking by controlling both electrode layout and switching voltages.
It is a purpose of this invention to generate electrostatic fields which provide a consistent and reliable tacking pressure, while accelerating the droplets to avoid deflection.
U.S. Pat. No. 6,079,814, which is assigned to the same assignee as the subject application and the disclosure of which is incorporated herein by reference, describes a printing system in which electrostatic fields are used to hold the paper (print medium) in place as it moves under the print head. In this instance, the electrostatic field is generated by a corona generating device such as a D-C scorotron. In the system of the ""814 patent, as shown in FIG. 1, a detacking A-C scorotron is positioned to remove charge from the paper after it leaves the printing station. A dielectric surface is provided under the print medium and is charged by the D-C scorotron. This charge generates an attraction force which accelerates the ink droplets in a direction perpendicular to the surface of the print medium. In addition it creates an electrostatic pressure to hold the print medium to its transport mechanism. The transport mechanism can be a belt, drum, or flat platen. The use of corona generating devices have the disadvantage of forming residual charges on the printed portion of the print medium which may cause deflection in adjacent printing operations.
It is a purpose of this invention to generate the tacking force and the attraction force without using a corona generating device.
U.S. Pat. No. 5,975,683 entitled xe2x80x9cElectric-Field Manipulation of Ejected Ink Drops in Printingxe2x80x9d and assigned to the same assignee as the present invention, discloses electrodes behind the recording medium and/or on the print head face to induce charges on the ejected ink droplets and accelerate them toward the recording medium. By appropriately controlling the electrostatic deflection of the ink droplets created by each column of actuators in the print head, the droplets are selectively directed to impact the recording medium at positions both left and right of a center position, so that each actuator can create up to three vertical print columns of spots on the recording medium, thus enhancing the printing resolution of the device.
It is a purpose of this invention to generate the tacking and attraction fields through the use of electrodes under the print medium and to save energy by optimizing the attraction field.
An array of electrodes is arranged under the print medium in a ink jet printing system to generate an electrostatic field for providing both an attraction field and a tacking field. The attraction field accelerates the droplets from the print head perpendicular to the print medium. The tacking field provides an electrostatic pressure to hold the print medium to its supporting surface as it moves through the print station.
The electrodes are arranged in adjacent pairs in a suitable dielectric material and are supplied with a first D-C voltage which is equal and opposite in each electrode of a pair. Adjoining electrodes are spaced to provide a suitable dielectric gap. A first D-C voltage generates the tacking field. A second D-C voltage is applied to the array at a significantly stepped up voltage from the first voltage, while the print head is maintained at ground potential. The voltage difference between the print head and the array provides a field assist to enhance the attraction field of the device and improve drop placement accuracy. A dielectric coating separates the electrode array from the print medium. By adjusting the first voltage to selected groups of electrodes, the printing of a swath is facilitated while avoiding the complete release of the print medium between swaths.